Insurgency. Early Access on Steam. <----- Get with the clicking.
It's in great shape already, games have shipped and went on to sell fantastically well in a far worse state. This is completely playable and a great fun right now.
As has already been said it has a bit of CS about it but with a greater range of weapons and attachments to buy between rounds. It has iron sights, prone and more realistic damage, movement and recoil. A smaller scale, more pick up and play version of RO2/Rising Storm in a modern setting isn't too far off, it even has a similarly excellent suppression mechanic. Pace-wise it would probably sit in between the two, running around like John J. will end in a bad way, so it pays to be a bit more cautious and methodical rather than living/dyng by reflexes alone.
It's definitely a more accessible game than RO2 but more meat than a lot of shooters.
tl;dr
Counter-Strike for adults.